1. Field of the Invention
The present invention relates to an apparatus and method for determining a failure of an EGR (exhaust gas recirculation) apparatus.
2. Description of the Related Art
The major ingredients of the exhaust gas discharged from a gasoline engine are carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxides (NOx). Nitrogen oxides are produced by the chemical reaction between nitrogen and oxygen contained in an air-fuel mixture under a high-temperature condition which takes place when the air-fuel mixture supplied to an engine burns. The majority of nitrogen oxides contained in the exhaust gas is nitric monoxide (NO). Even with the same air-fuel ratio of the air-fuel mixture, if the quality of inactive ingredients contained in the air-fuel mixture increases, the combustion temperature of the air-fuel mixture lowers with consequent reduction in the nitric monoxide produced when the air-fuel mixture burns.
Based on the fact described above, an EGR apparatus designed to cause part of exhaust gas to be returned to an induction system of an engine to thereby add the exhaust gas to an air-fuel mixture as an inactive ingredient is used for exhaust gas purification.
An EGR apparatus generally has an EGR passage for connecting an exhaust passage of an engine to an intake passage, a negative-pressure operated EGR valve disposed in the EGR passage to regulate the amount of exhaust gas introduced to the intake system (EGR amount), an electromagnetic control valve for causing the EGR valve to open and close by controlling negative pressure supplied from the intake passage to the negative pressure chamber of the EGR valve, and an electronic control unit (ECU) for determining a target EGR amount and controlling the drive of the electromagnetic control valve so as to attain the target EGR amount.
In the EGR configured as described above, the EGR valve itself may malfunction due to the seizure of the valve body of EGR valve, breakage of the diaphragm of EGR valve, and the like. Sometimes, breakage of the wire connecting the ECU to the electromagnetic control valve or poor contact of the connector may occur. If such a failure occurs in the EGR apparatus, it becomes impossible for the EGR apparatus to control the EGR amount, resulting in loss of the exhaust gas purifying function of the EGR apparatus.
As a method for diagnosing a failure of an EGR apparatus, "METHOD FOR DIAGNOSING A FAILURE OF AN EXHAUST GAS CIRCULATION CONTROLLER" which performs a failure diagnosis when an engine is running in a decelerated operation zone is disclosed in Japanese provisional patent publication no. H2-9937. According to this diagnosis method, to perform the failure diagnosis, when an engine is in a stable condition following the completion of warm-up, the EGR valve is temporarily changed over from an open state to a closed state, by which the exhaust gas circulates from the exhaust passage to the intake passage via the EGR passage. Then, a difference between the intake pressure developed immediately before the EGR and that developed during the EGR is detected. If the difference is below a preset value, then it is judged that a failure of the EGR apparatus has occurred.
In this diagnosis method, when the failure diagnosis executing condition becomes fulfilled again after the failure diagnosis executing condition becomes unfulfilled and the execution of EGR is stopped due to the change in vehicle operation state during the failure diagnosis, EGR is restarted immediately. Therefore, when failure diagnosis is executed during the vehicle running in an operating environment, for example, in an urban area where the vehicle operating condition is liable to be changed, the start and stop of EGR are repeated frequently. In this case, the increase in intake pressure caused by the execution of EGR and the decrease in intake pressure caused by the stop of EGR are repeated frequently, so that the engine speed and the engine output torque fluctuate. Therefore, the riding quality and drivability of vehicle are impaired.
Further, when it is judged that an EGR is faulty, failure diagnosis is sometimes performed continuously to prevent mistaken diagnosis. In this case, the aforementioned trouble appears more remarkably due to the immediate restart of EGR effected when the failure diagnosis executing condition is fulfilled again.